Microcellular plastic foam refers to a polymer that has been specially foamed to thereby create micro-pores or cells (also sometime referred to as bubbles). The common definition includes foams having an average cell size on the order of 10 microns in diameter, and typically ranging from about 0.1 to about 100 microns in diameter. In comparison, conventional plastic foams typically have an average cell diameter ranging from about 100 to 500 microns. Because the cells of microcellular plastic foams are so small, to the casual observer these specialty foams generally retain the appearance of a solid plastic.
Microcellular plastic foams can be used in many applications such as, for example, insulation, packaging, structures, and filters (D. Klempner and K. C. Fritsch, eds., Handbook of Polymeric Foams and Foam Technology, Hanser Publishers, Munich (1991)). Microcellular plastic foams have many unique characteristics. Specifically, they offer superior mechanical properties at reduced material weights and costs.
The process of making microcellular plastic foams has been developed based on a thermodynamic instability causing cell nucleation (J.E. Martini, SM Thesis, Department of Mech. Eng., MIT, Cambridge, Mass. (1981)). First, a polymer is saturated with a volatile foaming agent at a high pressure. Then, by means of a rapid pressure drop, the solubility of foaming agent impregnated within the polymer is decreased, and the polymer becomes supersaturated. The system is heated to soften the polymer matrix and a large number of cells are nucleated. The foaming agent diffuses both outwards and into a large number of small cells. Put differently, microcellular plastic foam may be produced by saturating a polymer with a gas or supercritical fluid and using a thermodynamic instability, typically a rapid pressure drop, to generate billions of cells per cubic centimeter (i.e., bubble density of greater than 108 cells per cubic centimeter) within the polymer matrix.
In the context of making a microcellular thermoplastic sheet from a solid gas impregnated thermoplastic sheet, the formation of microcellular bubbles is known to cause a substantial amount of in-plane and volume expansion of the plastic sheet as it is heated and becomes foamed (i.e., populated with microcellular bubbles). Indeed, the in-plane expansion ratio of typical gas impregnated thermoplastic sheets are up to about two (meaning that the in-plane area of a gas impregnated thermoplastic sheet can double as a result of microcellular foaming).
The process of heating a thermoplastic sheet to a working temperature and then forming it into a finished shape by means of heat or pressure is known as thermoforming. In the most common method of high-volume, continuous thermoforming of thin-gauge products, plastic sheet is fed from a roll or directly from an extruder into a set of indexing chains that incorporate pins, or spikes, that pierce the sheet and transport it through an oven (infrared, direct conduction, or convection) for heating to forming temperature. The heated sheet (commonly referred to as a “web”) then indexes into a form station where a mating mold and pressure-box close on the sheet, with vacuum then applied to remove trapped air and to pull the material into or onto the mold along with pressurized air to form the plastic to the detailed shape of the mold. (Plug-assists are typically used in addition to vacuum in the case of taller, deeper-draw formed parts in order to provide the needed material distribution and thicknesses in the finished parts.) After a short form cycle, a burst of reverse air pressure is actuated from the vacuum side of the mold as the form tooling opens, commonly referred to as air-eject, to break the vacuum and assist the formed parts off of, or out of, the mold. A stripper plate may also be utilized on the mold as it opens for ejection of more detailed parts or those with negative-draft, undercut areas. The sheet containing the formed parts then indexes into a trim station on the same machine, where a die cuts the parts from the remaining sheet web, or indexes into a separate trim press where the formed parts are trimmed. The sheet web remaining after the formed parts are trimmed is typically wound onto a take-up reel or fed into an inline granulator for recycling.
An exemplary thermoforming assembly in this regard is disclosed in U.S. Pat. No. 3,359,600 to Obrien et al. As disclosed in this patent, plastic web is carried by pins or clips that are attached to parallel chains that, in turn, are indexed through an oven and forming area by mechanical advancement of the chains. In this configuration, the plastic web generally sags as it expands in the oven and thus it no longer remains straight and level. As it sags, the center of the web moves closer to the bottom heating element and thus the entire web tends to heat unevenly. In addition, and as the width of the plastic web increases to accommodate larger thermoformers, the sag problem becomes even more pronounced. Indeed, it is believed that catenary sage generally grows as the square of the width of the web (meaning that for a given tension a 60 inch wide machine will have a web sag that is about 225% as much as the web sag of a comparable 40 inch wide machine). Another disadvantage of this exemplary pin-chain thermoforming system is that the attachment pins require up to about 1 inch of the plastic web width on each edge of the web (meaning that this pierced edge material is not available to make product). Yet another disadvantage is that the edges of plastic must be kept cool, otherwise the plastic will melt and the pins will pull loose (U.S. Pat. No. 3,359,600 discloses cooling the pin-chain in order to alleviate this problem).
Because the edges of the plastic web are specifically cooled (or shielded from heating) in exemplary pin-chain thermoforming systems (in order to prevent the pins from losing their grip on the plastic web), the plastic web is constrained from lengthwise expansion along its edges (in the machine direction). Thus, the plastic web not only tends to form a lengthwise “catenary sag” in the machine direction, but also tends to form “bagging” or “puckering” across the web due to the constrained linear expansion along its edges. Indeed for ordinary solid plastic webs, the thermal expansion coefficients typically range from 65-250×10−6/° C. (N. Rao and K. O'brien, Design Data for Plastic Engineers, Hanser Publishers, Munich (1998)). (The coefficient of linear thermal expansion (CLTE) measures the change in length per unit length of a material per unit change in temperature; expressed as in/in/° F. or cm/cm/° C., the CLTE is used to calculate the dimensional change resulting from thermal expansion.) Thus, and for example, plastic web conveyed through a conventional 40 inch wide pin-chain thermoforming system and heated from about 21° C. to 121° C. will expand from about 0.25 inches up to about 1.0 inch.
The above-noted disadvantages associated with conventional pin-chain thermoforming systems are even further exacerbated when the plastic web is a gas impregnated thermoplastic sheet (such as those disclosed by U.S. Pat. No. 5,684,055 to Kumar et al.). For gas impregnated thermoplastic sheets, the in-plane expansion ratio are up to two times in both the cross direction and machine direction when the web is heated and expanded during bubble formation. This large in-plane expansion renders conventional pin-chain thermoforming systems impractical for heating or otherwise expanding a gas impregnated thermoplastic sheet; the concomitant problems of web sagging, bagging, puckering, buckling and/or the formation of lengthwise “corrugations” are simply too great. Indeed, there is presently no means commercially available for satisfactorily heating and expanding, on a continuous basis, a solid (unfoamed) gas impregnated thermoplastic sheet or web into a substantially flat and corrugation-free microcellular foamed thermoplastic sheet or web.
Accordingly, there is still a need in the art for new and improved thermoforming systems, assemblies, and machines that enable the corrugation-free expansion of a gas impregnated thermoplastic sheet or web, as well as to methods relating thereto. The present invention fulfills these needs and provides for further related advantages.